Describing Motion Motion An object is in motion

Describing Motion

Motion • An object is in motion if its distance from another object is changing.

Reference Point • A reference point is a place or object used for comparison to determine if something is in motion. – An object is in motion if it changes position relative to a reference point

Relative Motion • Whether or not an object is in motion depends on the reference point.

Speed and Velocity

Speed • If you know the distance an object travels in a specific amount of time, you can calculate the speed of the object. • Speed of an object is the distance the object travels per unit of time

Calculating Speed • To calculate the speed of an object, divide the distance the object travels by the amount of time it takes to travel that distance.

Average Speed • Speed of most moving objects not constant • Average Speed is total distance (d) traveled divided by the total time (t).

Average Speed Example Total distance = 36 miles + 24 miles = 60 miles Total time = 3 hours + 1 hour = 4 hours Average speed (v) = d = 60 mi t 4 = 15 mph

Instantaneous Speed • Instantaneous speed is the rate at which an object is moving at a given instant in time.

Velocity • Velocity is speed in a given direction. – Like displacement, velocity is a vector – it has both magnitude (speed) and direction. • Changes in velocity may be due to changes in speed, changes in direction or both.

Graphing Speed • Slope – steepness of a line on a graph • Slope formula:

Acceleration

Changing Velocity • Acceleration is the rate at which velocity changes with time. – In science, acceleration refers to increasing speed, decreasing speed or changing direction.

Increasing Vs. Decreasing Speed • Whenever an object’s speed increases, the object accelerates. Just as objects can speed up, they also slow down, this is called negative acceleration.

Changing Direction • An object can accelerate even when it’s speed is constant – when it’s changing direction. – Circular motion

Calculating Acceleration • To determine the acceleration of an object, you must calculate its change in velocity per unit of time. Acceleration = Final velocity – Initial velocity Time

Graphing Acceleration • You can use both a speed – versus - time graph and a distance – versus - time graph to analyze the motion of an accelerating object.

Speed – Versus – Time Graph • The slope of a line on a speed – versus – time graph represents acceleration.

Distance – Versus – Time Graph • You can also show the motion of an accelerating object with a distance – versus – time graph.